Wheat weevil

Wheat weevil
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Order:	Coleoptera
Family:	Curculionidae
Subfamily:	Dryophthorinae
Tribe:	Litosomini
Genus:	Sitophilus
Species:	S. granarius
Binomial name
Sitophilus granarius (Linnaeus, 1758) [1]

The wheat weevil (Sitophilus granarius), also known as the grain weevil or granary weevil, occurs all over the world and is a common pest in many places. It can cause significant damage to harvested stored grains and may drastically decrease yields. The females lay many eggs and the larvae eat the inside of the grain kernels.

Identification

Adult wheat weevils are about 3–5 mm (0.12–0.20 in) long with elongated snouts and chewing mouthparts.^[2] Depending on the grain kernels, the sizes vary. In small grains, such as millet or grain sorghum, they are small in size, but are larger in maize (corn).^[3] The adults are a reddish-brown colour and lack distinguishing marks. Adult wheat weevils are not capable of flight. Larvae are legless, humpbacked, and white with a tan head. Weevils in the pupal stage have snouts like the adults.

Natural history

Lifecycle

Female wheat weevils lay between 36 and 254 eggs and usually one egg is deposited in each grain kernel. All larval stages and the pupal stage occur within the grain. The larvae feed inside the grain until pupation, after which they bore a hole out of the grain and emerge. They are rarely seen outside of the grain kernel. The lifecycle takes about 5 weeks in the summer, but may take up to 20 weeks in cooler temperatures.^[4] Adults can live up to 8 months after emerging.

Behaviour

Adult wheat weevils when threatened or disturbed will pull their legs close to their bodies and feign death. Female weevils can tell if a grain kernel has had an egg laid in it by another weevil.^[5] They avoid laying another egg in this grain. Females chew a hole, deposit an egg, and seal the hole with a gelatinous secretion. This may be how other females know the grain has an egg in it already. This ensures the young will survive and produce another generation. One pair of weevils may produce up to 6,000 offspring per year.

Human impact

Wheat weevils are a pest of wheat, oats, rye, barley, rice and corn. Wheat weevils cause an unknown amount of damage worldwide because keeping track of so much information is difficult, especially in places where the grain harvests are not measured. They are hard to detect and usually all of the grain in an infested storage facility must be destroyed. Many methods attempt to get rid of the wheat weevil, such as pesticides, different methods of masking the odour of the grain with unpleasant scents, and introducing other organisms that are predators of the weevils.^[6]

Prevention and control

Sanitation and inspection are the keys to prevent the infestation. Grains should be stored in preferably metallic (cardboard, even fortified, is easily drilled through) containers with tight lids in a refrigerator or a freezer, and should be purchased in small quantities. If any suspicion has arisen, carefully examine the grains for adult insects or holes in the grain kernels. Another method is to immerse them in water. If they float to the surface, it is a good indication of infestation. Even if identified early, disposal may be the only effective solution.

Deltamethrin powder (WP) is another solution to weevil infestation in grains.

See also

• Rice weevil (Sitophilus oryzae)
• Maize weevil (Sitophilus zeamais)
• Lixus concavus, the rhubarb curculio weevil

References

External links

Wikispecies has information related to: Sitophilus granarius

Wikimedia Commons has media related to Sitophilus granarius.

• Granary and rice weevils
• Sitophilus granarius in the CalPhotos Photo Database, University of California, Berkeley